The technique of eliminating a vascular stenosis by dilating a balloon on a catheter placed within the stenosis was developed by Dr. Andreas Gruntzig. The first marketable catheters for angioplasty were "fixed wire" catheters, in which a core or guidewire was fixed within the catheter to stiffen it so that it could be pushed into position in the vascular system.
Dr. John Simpson and Dr. Edward Robert subsequently developed an "over-the-wire" catheter in which a guidewire was slidably placed within a lumen of the catheter. This system provided reasonably easy placement of the catheter because the guidewire was first positioned beyond the stenosis and the catheter was then slid into place over it. Although over-the-wire catheters generally have a larger profile than fixed wire catheters, the guidewire can be much more easily positioned in the vascular system than a fixed wire catheter.
Both over-the-wire and fixed wire catheters are usually made using polymer tubing to form the catheter body. In some catheters, however, the catheter shaft is formed of a spring coil (a helically wound wire) jacketed on the outside or inside so that it is sealed to form a lumen. For example, U.S. Pat. Nos. 4,976,689, 4,944,740, 4,917,666 and 4,723,936 issued to the assignee of the present invention describe such catheters. Although more expensive and more complicated to make than polymer catheters, spring coil catheters have certain advantages. They allow flexibility in the catheter while providing greater axial stiffness than a typical polymer extrusion. As a result, the catheter is very "pushable", i.e., axial force at one end is transmitted to the other end. In addition, kinking of the catheter as it bends around curves is minimized. The use of flat wire rather than round wire is preferred because it has greater resistance to compression and less tendency to deform.
An advantage of over-the-wire catheters is that if a catheter has to be exchanged for a larger or smaller catheter, the guidewire can be left in place and the catheter withdrawn over it and another catheter slid into place over it. A difficulty with the exchange procedure is that it is difficult to keep the guidewire in place, because removing the catheter requires removal of the guidewire and subsequent recrossing of the stenosis. To avoid this problem, very long "exchange" guidewires, more than twice the length of the catheter, are used so that they can be separately held in place while the catheter is withdrawn. In addition, shorter guidewires have been made, which are lengthened by attachment of an extension wire during the exchange process in order to render them the length of a long exchange wire. Unfortunately, such long guidewires and extension wires require an additional person to hold the guidewire during the catheterization process and are somewhat difficult to use.
This problem was solved by the development of catheters which have shorter guidewire lumens, so that the guidewire exits from the catheter closer to the balloon than to the proximal end of the catheter. Thus the guidewire can be anchored or held by the physician as he or she removes the catheter from the body and the exchange occurring over the shorter guidewire lumen.
One version of such a catheter is shown in U.S. Pat. No. 4,762,129 (and B1 4,762,129) issued to Bonzel, where the guidewire lumen passes through the balloon and exits immediately proximal to the balloon. The guidewire lumen and inflation lumen are of a "bilumen" or "biaxial" configuration in which the guidewire lumen runs parallel to the inflation lumen. A similar system is shown in U.S. Pat. No. 4,748,982 issued to Horzewski, et al., and in U.S. Pat. No. 4,988,356 issued to Crittenden, in which the guidewire lumen, which runs parallel to the inflation lumen, contains a slit extending its length so that the guidewire can be removed from the lumen through the slit at a point immediately proximal to the balloon.
These bilumen designs can be relatively easy to manufacture because they can be made from a single extrusion of the shaft and guidewire lumen together. In addition, they allow use of a slit guidewire lumen. Sometimes, however, they have a larger profile than might be desired and poor guidewire movement.
Examples of bilumen rapid exchange catheters on the market are ACS' Alpha.TM. catheter and ACS' RX.TM. catheter. In the Alpha.TM. catheter, a hypotube (stainless steel tube) forms the proximal end of the catheter and a bilumen extrusion the distal portion. The bilumen portion is slit so that the guidewire can be removed from it at varying positions as shown in the Horzewski, et al., patent mentioned above. In the RX.TM. catheter, the entire catheter is a single bilumen extrusion, the proximal portion of which contains a core wire. A side entry is cut into the guidewire lumen near the balloon.
In rapid exchange catheter designs such as those in Yock, U.S. Pat. Nos. 5,040,548 and 5,061,273, the short guidewire lumen is coaxial with respect to the inflation lumen, but exits (or enters) in a side port at least 10 centimeters from the distal tip of the catheter. (The Yock disclosure suggests a lumen of 10 or more centimeters; in catheters on the market, the coaxial lumen varies from about 9 to about 35 centimeters in length.) Coaxial construction has provided certain advantages such as smaller profile catheters and better guidwire movement.
However, in these catheters, the construction of the distal guidewire entry area or "transition region" has posed a challenge. The inflation lumen must be isolated from the distal port to prevent exit of the inflation fluid to the exterior. In some designs, the transition region is not strong enough to avoid distal kinking. In others, abrupt changes in stiffness from one part of the catheter to another may occur. In yet others, the transition region may be too stiff, preventing its placement in the coronary arteries.
An example of a coaxial rapid exchange catheter on the market is Schneider's Piccolino.TM.. In this catheter, the entire inflation lumen appears to be formed of one piece, and a core wire extends through the proximal portion, through the transition region and into the distal portion. The guidewire lumen is located in the distal end of the inflation lumen and appears to be fused into position in the transition area. An entry is cut into the proximal end of the guidewire and adjacent fused area.
In SciMed's Express.TM. catheter, a hypotube forms the proximal segment and a separate hypotube segment formed into a crescent shape is attached to the distal end of the proximal hypotube, creating a trough in which the guidewire lumen is located. A short coil jacketed by the inflation lumen surrounds the guidewire lumen, reinforcing the transition. The remaining distal segment of the catheter is made of standard coaxial extrusions.
It would be desirable to develop a catheter which allows rapid exchange, has the benefits of a coaxial guidwire lumen, has the advantages of a spring coil design, and which can be designed for appropriate but varying flexibility along the length of the catheter, without abrupt changes in stiffness, or an undesirably stiff transition region.